Kernel interface

Warning: if your goal is just to write a game/program use higher level libraries/interfaces like SDL, Qt, X or similar for your own good (portability). It may be impossible to retain this layout on some major hardware revision, let's say Pandora 3000, and your program will break. Also this requires some level of Linux programming knowledge.

In case you need to write low level code (you can't/don't want to use high level libs like SDL), you can use kernel interface. This is the recommended way to access hardware (as opposed to GP2X style of accessing chip registers by mmap'ing /dev/mem), because in case hardware changes are needed in future, they could be handled by kernel and all programs would still work. It also should allow several programs to work at the same time and should be more stable.

Input

Buttons, keypad, touchscreen and nubs are all exposed through Linux event interface (EVDEV). All devices are represented by /dev/input/eventX files, which can be opened, read and queried (using ioctl calls). The reads can be synchronous (the read will only return when user does something, like presses the button), or asynchronous (the system will report what changed since the last time you asked).

Warning: don't hardcode device filenames in your program! For example, currently /dev/input/event2 represents game buttons, but in future it may become touchscreen. Scan input device names instead, example:

for (i = 0; 1; i++)
{
  sprintf(name, "/dev/input/event%i", i);
  fd = open(name, O_RDONLY);
  if (fd < 0) break; /* no more devices */
  ioctl(fd, EVIOCGNAME(sizeof(name)), name);
  if (strcmp(name, "gpio-keys") == 0)
    return fd; /* found the buttons! */
  close(fd); /* we don't need this device */
}

List of device names and events they send:

name	description	event.type	event.code	event.value
keypad	keypad	EV_KEY	KEY_0...KEY_Z, KEY_BACKSPACE, KEY_LEFTSHIFT, KEY_SPACE, KEY_ENTER, KEY_COMMA, KEY_DOT, KEY_FN	0 - released, 1 - pressed, 2 - autorepeat event
gpio-keys	game buttons	EV_KEY	KEY_UP, KEY_DOWN, KEY_LEFT, KEY_RIGHT, KEY_MENU (Pandora button), KEY_LEFTALT (Start), KEY_LEFTCTRL (Select), KEY_KP1 (Y/North), KEY_KP2 (A/East), KEY_KP3 (X/South), KEY_KP4 (B/West), KEY_KP5 (Shoulder L), KEY_KP6 (Shoulder R), KEY_KP7 (Shoulder L2), KEY_KP8 (Shoulder R2), KEY_COFFEE (Hold)	0 - released, 1 - pressed
gpio-keys	lid state	EV_SW	SW_LID	0 - closing, 1 - opening
touchscreen	touchscreen	EV_ABS	ABS_X, ABS_Y, ABS_PRESSURE	varies, use calibration data
nub0	left nub	EV_ABS	ABS_X, ABS_Y	-256 (Left/Up) ...0... +256 (Right/Down)
nub1	right nub	EV_ABS	ABS_X, ABS_Y	-256 (Left/Up) ...0... +256 (Right/Down)

Sample code:
evtest.c op_test_inputs.c

Touchscreen

Event interface returns uncalibrated values directly from driver, so you need to use tslib or manage calibration yourself (using data from /etc/pointercal).

Sound

Pandora uses ALSA, but it has OSS emulation enabled too, so GP2X code should work.

Video

Architecture

Framebuffer device (/dev/fbX) is supported. There are 3 framebuffers available (/dev/fb0, /dev/fb1 and /dev/fb2), which represent 3 graphics/video layers on OMAP3 by default (but can be reconfigured). Only /dev/fb0 is enabled by default.

OMAP3 display subsystem is controlled by a driver known as DSS2, which has various controls available on /sys/devices/platform/omapdss/ (but they are not meant to be changed by programs, so they are root writable only). The driver exposes 3 layers (called overlays) and 2 displays. Overlays 1 and 2 can perform hardware scaling on the fly using 5-tap poly-phase filter, overlay0 can not. Displays 0 and 1 represent LCD and TV respectively. By default the 3 framebuffers (/dev/fbX) are redirected to 3 overlays, which all output to the LCD. This configuration is not meant to be changed by programs, only firmware should manage these.

Basic usage

framebuffer interface

Framebuffers can be accessed Linux fbdev interface:

fbdev = open("/dev/fb0", O_RDONLY);
buffer = mmap(0, 800*480*2, PROT_WRITE, MAP_SHARED, fbdev, 0);

(this is basic example, no error checks)

the returned pointer can be used to draw on the screen.

Be sure to #include <linux/fb.h> to get access to the FB device ioctl interface, and <sys/ioctl.h> for access to ioctl itself.

double buffering

This can be achieved using FBIOPAN_DISPLAY ioctl system call. For this you need to mmap framebuffer of double size

buffer1 = mmap(0, 800*480*2 * 2, PROT_WRITE, MAP_SHARED, fbdev, 0);
buffer2 = (char *)mem + 800*480*2;

Then to display buffer2 you would call:

struct fb_var_screeninfo fbvar;
ioctl(fbdev, FBIOGET_VSCREENINFO, &fbvar);
fbvar.yoffset = 480;
ioctl(fbdev, FBIOPAN_DISPLAY, &fbvar);

going back to buffer1 would be repeating above with fbvar.yoffset = 0. Tripple or quad buffering can be implemented using the same technique.

vertical sync

Linux has standard FBIO_WAITFORVSYNC for this:

int arg = 0;
ioctl(fbdev, FBIO_WAITFORVSYNC, &arg);

be sure to pass argument value 0 or it will not work.

Currently FBIO_WAITFORVSYNC is not defined in <linux/fb.h>, although this is in the process of modification. For now, define in the following manner:

#ifndef FBIO_WAITFORVSYNC
  #define FBIO_WAITFORVSYNC _IOW('F', 0x20, __u32)
#endif

hardware scaling

Overlay1 (/dev/fb1) can be used to achieve hardware scaling. Technically overlay2 (fb2) can be used for this too, but it is planned to be used by the system for TV-out functionality, so don't use it. The overlay is configured using series of standard and OMAP specific ioctl calls, but the system ships with some tools to achieve this from scripts too. This way the framebuffer can be set up for some arbitrary size (say 320x240) and can output to LCD as 800x480 with hardware scaling. Here is an example script:

ofbset -fb /dev/fb1 -pos 0 0 -size 800 480 -mem 307200 -en 1
fbset -fb /dev/fb1 -g 320 240 320 480 16

./your_app_here

ofbset -fb /dev/fb1 -pos 0 0 -size 0 0 -mem 0 -en 0

What it does:

• allocates OMAP DSS layer, asks video output to be 800x480 at position 0,0 (could set it to 640x480 at 80,0 instead to get centered 2x scaling of 320x240). 307200 bytes of video memory are allocated for 2 320x240 16bpp screens (for doublebuffering).
• sets video mode to 320x240@16bpp, virtual resolution 320x480 for doublebuffering.
• runs your app
• cleans the video layer on exit

Now the program can act as if works with 16bpp screen.

LEDs and backlight

The LEDs can be controlled via /sys/class/leds/, and then a file [1]:

• pandora::sd1
• pandora::sd2
• pandora::charger
• pandora::power
• pandora::bluetooth
• pandora::wifi
• pandora::keypad_bl

Backlight can be controlled via /sys/class/backlight/.

Misc

Some things can be controlled through files in /proc/pandora/:

• /proc/pandora/cpu_mhz_max - if cpufreq is enabled, sets maximum allowed cpu clock, if not, just sets CPU clock to value supplied (echo 600 > /proc/pandora/cpu_mhz_max). Might also just use cpufreq parameters themselves.

more to come..